A Tug-of-War Model Explains the Saltatory Sperm Cell Movement in Arabidopsis thaliana Pollen Tubes by Kinesins With Calponin Homology Domain

Abstract

Upon pollination, two sperm cells are transported inside the growing pollen tube toward the apex. One sperm cell fertilizes the egg cell to form the zygote, while the other fuses with the two polar nuclei to form the triploid endosperm. In Arabidopsis thaliana, the transport of the two sperm cells is characterized by sequential forward and backward movements with intermediate pauses. Until now, it is under debate which components of the plant cytoskeleton govern this mechanism. The sperm cells are interconnected and linked to the vegetative nucleus via a cytoplasmic projection, thus forming the male germ unit. This led to the common hypothesis that the vegetative nucleus is actively transported via myosin motors along actin cables while pulling along the sperm cells as passive cargo. In this study, however, we show that upon occasional germ unit disassembly, the sperm cells are transported independently and still follow the same bidirectional movement pattern. Moreover, we found that the net movement of sperm cells results from a combination of both longer and faster runs toward the pollen tube apex. We propose that the observed saltatory movement can be explained by the function of kinesins with calponin homology domain (KCH). This subgroup of the kinesin-14 family actively links actin filaments and microtubules. Based on KCH's specific properties derived from in vitro experiments, we built a tug-of-war model that could reproduce the characteristic sperm cell movement in pollen tubes.

Removal of the endoplasma membrane upon sperm cell activation after pollen tube discharge

“…WT pollen tubes maintained an eyeglass-like IVPM morphology throughout the growth phase ( Figures 1G ). In the pollen tube, the MGU is continuously exposed to mechanical stresses: stretching, twisting, and moving back and forth in intense protoplasmic flow ( Schattner et al., 2020 ). The MGU structure, particularly the tail-like connection, becomes fragile after reducing cellular stiffness by disrupting the coordination of motility between a pair of sperm cells and the vegetative nucleus ( Zhou and Meier, 2014 ; Motomura et al., 2021 ).…”

A combined transcriptome - miRNAome approach revealed that a kinesin gene is differentially targeted by a novel miRNA in an apomictic genotype of Eragrostis curvula

“…KCHs are most abundant in developing tissues and are thought to participate in cell division and elongation (Frey et al, 2010). Identified KCHs showed to be mostly expressed in mature pollen, being proposed that they are the main actors for the movement of sperm cells in the pollen tube (Schattner et al, 2021). The expression of OsKCH1 was quantified in rice, revealing that the highest abundance of transcripts was found in primary leaf, primary root, and the developing flower (Frey et al, 2010).…”

Distinct functions of microtubules and actin filaments in the transportation of the male germ unit in pollen

“…An immunofluorescence assay using heterogeneous anti-myosin antibodies revealed that myosin I was localized to the surface of generative cells and VN in lily and tobacco pollen tubes 21 , indicating the possible participation of the actin cytoskeleton in MGU migration. Moreover, a recent study observed saltatory SC movement in Arabidopsis pollen tubes 22 . The authors assumed that SC movement was mediated by kinesins with a calponin homology domain (KCH) that can bind both actin filaments and microtubules 22 .…”

“…Moreover, a recent study observed saltatory SC movement in Arabidopsis pollen tubes 22 . The authors assumed that SC movement was mediated by kinesins with a calponin homology domain (KCH) that can bind both actin filaments and microtubules 22 . Collectively, these studies implied that microtubules and actin filaments may both be involved in MGU migration, but their specific functions and how they coordinate have not been studied thus far.…”